1. Technical Field
The invention relates generally to precision measurement instruments, and particularly to absolute position encoder scales that may be utilized in handheld instruments such as calipers.
2. Description of the Related Art
Various movement or position transducers are currently available, such as optical, capacitive, magnetic, and inductive transducers. These transducers often involve placing a transmitter and a receiver in various geometric configurations to measure movement between two members of the transducer, typically including a read head and a scale. One drawback of certain optical, capacitive, and magnetic transducers is that they tend to be sensitive to contamination. Therefore, using such transducers in most manufacturing or shop environments is impractical. Using such transducers in a shop environment requires expensive and sometimes unreliable environmental seals or other methods of encapsulating the transducer to keep dust, oils, and ferromagnetic particles from contaminating the transducer.
U.S. Pat. No. 6,011,389 (the '389 patent), which is hereby incorporated herein by reference in its entirety, describes an induced current position transducer usable in high accuracy applications. U.S. Pat. No. 5,973,494 (the '494 patent) and U.S. Pat. No. 6,002,250 (the '250 patent), which are each hereby incorporated herein by reference in their entireties, describe incremental position inductive calipers and linear scales, including signal generating and processing circuits. U.S. Pat. Nos. 5,886,519, 5,841,274, and 5,894,678, which are each hereby incorporated herein by reference in their entireties, describe absolute position inductive calipers and electronic tape measures using this induced current transducer. As described in these patents, this induced current transducer is readily manufactured using known printed circuit board technology. This transducer system is also generally immune to contamination by particles, including ferromagnetic particles, oil, water, and other fluids.
As noted above, different implementations of the induced current transducer (as well as the previously noted optical, capacitive, and magnetic transducers) may be implemented as either incremental or absolute position encoders. In general, incremental position encoders utilize a scale structure that allows the displacement of a read head relative to a scale to be determined by accumulating incremental units of displacement, starting from an initial point along the scale. Such encoders are suitable for certain applications, particularly those where line power is available. However, in certain applications, such as those where encoders are used in low power consumption devices, it is more desirable to use absolute position encoders. Absolute position encoders provide a unique output signal, or combination of signals, at each position along a scale. They do not require continuous accumulation of incremental displacements in order to identify a position. Thus, absolute position encoders allow various power conservation schemes. A variety of absolute position encoders are known, using various optical, capacitive, magnetic and inductive technologies, such as those described above.
In addition to the '519; '274, and '678 patents described above for the absolute induced current transducer, U.S. Pat. Nos. 3,882,482, 5,965,879, 5,279,044, 5,237,391, 5,442,166, 4,964,727, 4,414,754, 4,109,389, 5,773,820, and 5,010,655, also disclose various encoder configurations and/or signal processing techniques relevant to absolute encoders, and are each hereby incorporated herein by reference in their entirety. However, many of these disclosed systems fail to teach configurations which provide certain combinations of compact size, high resolution, cost and robustness, including an ability to be generally immune to contamination by particles (e.g., ferromagnetic particles, oil, water, and other fluids) desired by users of absolute encoders. Improved configurations of absolute encoders that provide such combinations would be desirable.